Estimation of Irregular Underground-Structure From Seismic Ground Motions

Abstract

Phinney's method was extended so as to be able to extract the seismic effects of the underground structures containing irregular interfaces. The spectral ratios of the vertical component to the horizontal one of P- and S-wave motions were expressed as a function of only the propagation-media parameters. The underground structure model was iteratively determined by finding out the theoretical values consistent with the observed ones. The validity and the range of applicability of this method were examined by using the seismic data observed at two sites: one obtained with the array observation in the area across the buried fault (Obaku fault in the south-eastern side of Kyoto basin) and the other, at a single station near a seaside (Urakawa observatory at the southern coast of central Hokkaido). The spectral ratios of the seismic waves with the array observation show the spatial variation, being related to the lateral variation of the layer thickness. Further, even with a single station observation from the azimuthal variation of the spectral ratios, we can estimate the irregularities of the underground structure.